1. Field of the Invention
The present invention relates in general to a switching mode power supply (referred to hereinafter as SMPS) for supplying a stable operating voltage according to power consumption of a load, and more particularly to an initial drive voltage supply control circuit for the SMPS which is capable of initially supplying an output voltage from a rectifier as an initial drive voltage to an output power controller which generates a switching pulse width modulation (referred to hereinafter as PWM) signal and then blocking the output voltage from the rectifier to the output power controller.
2. Description of the Prior Art
Switching mode power supplies having an initial drive voltage control circuit are generally known. An example of such a SMPS is proved by U.S. Pat. No. 5,452,196 to Steffen Lehr et al. entitled Start-Up Circuit For A Switch Mode Power Supply.
Additionally, it is known that a SMPS rectifies and smooths a commercial alternating current (AC) voltage to convert it into a direct current (DC) voltage. Then, the SMPS adjusts an output duty ratio of the DC voltage by means of a PWM circuit to supply a desired operating voltage to a load through a switching transformer. Also, a variation in the operating voltage supplied to the load is fed back to the PWM circuit to adjust a duty ratio of a DC voltage to the switching transformer, so as to stabilize the operating voltage to the load.
Noticeably, the PWM circuit is initially operated in response to the initial drive voltage from an initial drive voltage supply unit provided with an initial drive voltage supply resistor. Thereafter, at the normal state, the PWM circuit is operated in response to a drive voltage from a drive voltage supply unit which is generated on the basis of the voltage induced in the tertiary coil of a switching transformer.
However, both of the above-mentioned switching mode power supplies have a disadvantage in that current continuously flows to an initial drive voltage supply resistor even at the normal state after the initial drive voltage is initially supplied to the PWM circuit through the initial drive voltage supply resistor. Because current continuously flows to the initial drive voltage supply resistor even at the normal state, unnecessary power is consumed. Further, the initial drive voltage supply resistor must have a high power capacity to reduce its overheat resulting from a self-overcurrent.